The use of switching converters is well known for converting an input voltage to a controlled output voltage for supply of a consumer. The power input, and consequently the output voltage, is controlled using a switch driven by a pulse width-modulated drive signal, and which is normally connected in series with an inductive energy storage element. To generate the pulse width-modulated signal, current mode switching converters (CM converters) use a feedback or control signal that varies with the output voltage, and a current measuring signal that is dependent on the current through the switch.
An example of a drive circuit for a switch in a CM converter of this kind is described in “CoolSET™-F2”, Datasheet V4.4, December 2003 from Infineon Technologies AG, Munich. This drive circuit has a so-called soft start function, which is implemented using a capacitor connected to the drive circuit. When restarting the switching converter, when the output voltage is still zero and the capacitor is not charged, the capacitor is charged under defined conditions to a specified voltage to create a limiting signal with a rising signal characteristic. The limiting signal is used during the startup phase to limit the current through the switch controlling the power input. To this end, a current measuring signal dependent on the current through the switch is compared with the limiting signal, and the switch is opened independently of the output voltage whenever the current measuring signal—which rises after activation of the switch—reaches the value of the limiting signal. Due to the rise in the level of the limiting signal over time, the current limiting value also rises over time and the activation periods of the switch are extended, which has the effect of producing a ‘gentle’ startup phase for the switching converter. The current measuring signal compared with the limiting signal during startup and with the feedback signal during normal operation may result from the voltage across a current sense resistor plus an offset to achieve improved startup conditions.
A drive circuit with soft start functionality for a switch in a CM switching converter is also described in the “Application Note 4116, A Fairchild Power Switch (FPS) based on Switched Mode Power Supply for LCD Monitor Use”, Rev. 1.0.0, 2002, Fairchild Semiconductor Corporation. The soft start function is also implemented in this instance using a capacitor that is charged after activation of the switching converter to produce a rising limiting signal. This drive circuit also has an overload protective circuit that prevents activation of the switch whenever overloading is detected. The overload detection functions by comparing the feedback signal with a specified limit value. An overload situation is assumed to exist if the feedback signal exceeds the limit value. To prevent the protection circuit from being triggered by brief rises in the feedback signal, there is a further capacitor, which is connected to the feedback branch. This capacitor provides a delay between the rise in the feedback signal and the activation of the protection circuit.
The disadvantage of this method is that the presence of the capacitor in the feedback path affects the control properties of the overall system and that the additional capacitor has to be designed as an external component, which represents a cost factor that has to be considered.
A drive circuit for a switch in a CM switching converter is also described in “TOP 242-250, TOPSwitch™-GX Family”, March 2004, Power Integrations. This drive circuit allows the switching frequency of the pulse width-modulated signal to be varied in order to enhance the electromagnetic compatibility. The switching frequency for the circuit varies between 128 kHz and 136 kHz. The frequency at which the switches take place is 250 Hz.